Sound amplifier and reproducing apparatus



Nov. 24, 1 93]. w THOMPSQN 1,833,762

SOUND AMPLIFIER AND REPRODUCING APPARATUS Filed May 10, 1928 Fig. I.

I LOUIS W. THOMPSON,

1 represented b quency.

\ the apparatus UNITED, s'r ras- PATENT: oar-ice or scnnnac'ranv, NEW YORK, assreiion TO GENERAL mnc'rarc COMPANY, A. CORPORATION OF NEW YORK SOUND AMPL'IIIIEB AND REPBDDUCING APPABT'Il'S Application filed Kay 10, 1928. 'SeriaI No. 276,742.

The present invention relates to apparatus for amplifying and reproducing sounds electrical means and an apparatus of this character whic 7 is energized in part at least from a source of alternating current.

Such an apparatus may, for example, be the audio frequency ampli-' I fier and soun reproducer of a broadcast or other receiver employing electron-discharge devices of the high vacuum type, the filament or cathode circuits of which are supplied with low 'voltage alternating current from asource of the usual commercial low fre- In a sound amplifying and reproducing apparatus of this character, the pulsations of the low frequency source may be transmitted through the amplifying portion of and energize the reproducing device or loud speaker or modulate the current flow therein 'sufliciently to produce an b more particularl i cation in certain details of the apparatus.

audible hum. This hum is often faint, and

noticeable only when the apparatus is en- 5 ergizedbut not operating to amplify and reproduce the signals or sounds normally carried by it. The hum is, however, annoyingin when of such'magnitude as to be audibly 3 superimposed upon or mingled with the nor- 5 nating currents ina sound amplifying and apparatus is avoided by the promal slgnals delivered by the reproducer or speaker.

In accordance .with my invention the undesirable hum effect of low frequency alterreproducing vision of an improved apparatus'and method of operation whereby a neutralizing current r of suitable phase and frequency is superimposed on the operating current of theredrawings and its scope will be producing apparatus.

. My invention will be better understood from the following description when considered in connection with the accompanying pointed out in the appended claims. 7 a

In the drawings, 1 is a wiring dia gram of a sound amplifying and reproducing apparatus which includes electron-discharge devices having their cath e circuits supitself and undesirable, particularly.

' of any sound plied with alternating current, and means arranged in accordance with theinvention for eliminating alternating current hum in the reproducer; and Fig. 2 illustrates a modifi- Referring to Fig. 1, 5 and 6 are electrondischarge devices connected in any suitable mannerfor sound amplification, as for example by audio frequency transformers 7, 8 and 9, between a signal or sound input circuit 10 and the armature circuitll of a reproducer or loud speaker 12.

The plate or anode supply is taken from any suitable source, such-as batteries 13, and the grid and plate return to the cathodes or filaments is carried through midtap resistors v or stabilizers 141 across the filament circuit 15 to reach the electrical center of the filament connection with alternating current supply. Filament current is taken from a suitable source of alternating current, such as a house lighting circuit 16 of the usual voltage and low frequency through a step down transformer 17 The loud speaker 12 may be of any suitable type and in the present exampleis represented as being of the .dynamic cone type having an armature coil 18 connected with the output circuit 11 of the amplifier. Coil 18' represents the actuating coil or winding reproducing device. The field of the loud speaker of the present example is' provided by a magnet indicated at 19 and a coil 20 energized from a constant voltage source, such as a battery 21. A The amplifier and reproducer. arrangement above described is shown only by way of example as illustrating an apparatus to whichthe method and means of the present invention is applicable. It consists essentially, for purposes of illustratin and describing the invention, of an amph er for an electrical sound reproducer, certain of the circuits of which amplifier are supplied with alternating current.

In the operation of the apparatus a small operating winding of the loud speaker along with the signal or operating current. n order to neutralize the hum produced by this amount of alternating current gets into the 24. In the stray current in theoperating winding of the loud speaker a neutralizing current taken from the low frequency supply circuit 16 U through a suitable step down transformer 22 is fed into the loild speaker orampli-fier output circuit 11 at such a phase angle and magnitude with respect to therhum or stray current as to exactly neutralizethe effect of the latter.

The means employed in the present exam-- secondary of a transformer 22, through a variable inductance- 28.

Inductance 28 represents any suitable reactance means in the neutralizing current supply circuit which may be varied to vary the phase angle of the neutralizing current whereby it will oppose the hum current. For 5 example it may comprise an inductance 29 and capacitance 30 connected in shunt as shown in Fig. 2. As shown, the -capacitance is variable, although either or both may be made variable to provide the necessary reactance to bring the neutralizing current into proper'phase relation with the hum or stray current in the loud speaker toneutralize the effect of the latter current.

The magnitude of the neutralizing current varied by means of the taps 27 on, resistor present example one tap is made variable with respect to the other to increase and decrease the magnitude of the, neutral izing current, an increase being efiected as it is moved away from the other along the resistor.

While but one neutralizing circuit 26 is generally sulficient to eliminate the hum, it is obvious that others similar manner, each tapped into resistor 24, with a variable-reactance in circuit, as shown for circuit 26, to neutralize a plurality of hum or stray currents or harmonics thereof.

From the foregoing description it will be seen that the neutralizing current is taken preferably from the same source as the hum 0r stray current, so that having a common source, variations in the voltage or frequency of the source do not appreciably affect'the neutralized condition in the loud speaker once it has been attained. This method of eliminating hum and undesirable noises in the reproducer is readily carried out by simple means which are easily applied to any amplifier and reproducer, as by the resistor "in the reproducer or m reactance controlled supply circuit connectoutput circuit and a in the resistor with the sources of hum.

dvhilethe invention is not limited to these means, the use of a' reactance and resistance means in a circuit connecting the source of 35 stray current with the operating winding or tralizing current may be connected in a reproducer,

circuit of the reproducer as shown is at present considered to source of'the neutralizing current automati- V cally compensates for variations in the nitude and frequency of the hum or current since the neutralizing current magwill

vary correspondingly and tend to maintain the neutralization substantially constant. However, any other suitable source of neumay be used.

What I claim as new and desire to secure by Letters Patent of the United States, is: 1. The method of neutralizing the "hum effect of stray alternating'current in'a sound reproducer operated in connection with an alternating current energized amplifier, said stray current being present in the signal cur rent supplied to said reproducer from said amplifier, which comprises supplying alternatmg current to the armature winding of the, reproducer fromthe same alternating current source as the amplifier at a controlled value and phase angle with respect 'to' saidstray alternating current.

2. .The method of neutralizing the hum effect of stray alternating current in a sound reproducer, said stray current being present in the signal current supplied to said reproducer, which comprises supplying alternating current of a frequency corresponding to thatof the stray current to the armature winding of the reproducer at a controlled be the preferred form of the Y invention because of its simplicity, and eflecstray' value and 1 phase angle with respect to said stray alteranting current.

3. The method of neutralizing the hum effeet of stray alternating current in a sound reproducer, said stray current being present in the signal current supplied to said reproducer, which comprises supplying alternating current from the source of the stray alternating current to the armature winding, of the reproducer ata controlled 'value and phase angle with respect to said stray alternating current.

4. The method effect of stray alternating current in a sound ducer, means for varying the phase angle of 130 of neutralizing the humsaid stray currentbeing present "1n the signal current supplied to said reprovalue and phase angle 1 7 said current, and means for varying the magnitude of said current.

6. In combination, an electrical sound amplifying apparatus having an'output circuit, an alternating current supply circuit connected with said amplifying apparatus, a reproducer connected with said output circuit, and means for supplying a neutralizing alternating current to said output circuit independently of the amplifying apparatus.

7. In combination, an electrlcal sound amplifying apparatus, having an output circuit, an alternating currentsupply circuit connected to the cathode heating circuit of said amplifying apparatus, and means for supplying a neutralizing alternating current to said output circuit from said alternating current supply circuit independently of the amplifying apparatus. I 8. In combination, an electrical sound amplifying apparatus, having an output. circuit, an alternating current supply circuitconnected with said amplifying apparatus,

a reproducer connected with said output circuit, means for supplying a neutralizing alternating current to the output circuit and reproducer independently of the amplifying apparatus, said means comprising a. resistor connected in said output circuit, an alternating current supply circuit connected with the resistor, and a variable reactance in said last-named circuit.

9. In combination, an electrical sound am-, plifying apparatus having an output circuit, an alternating current supply circuit connected with said amplifying apparatus, a reproducer connected with said output circuit, means for supplying a neutralizing alternating current to the output circuit and reproducer from the alternating current supply circuit independently of the amplifying apparatus, said means comprising a resistor connected in said output circuit, said resistor having spaced taps, at least one of which is movable along the resistor, a neutralizing current circuit connecting the alternating current supply circuit with the resistor at said spaced taps, and a variable reactance in said neutralizing current circuit.

10. The combination with an electrical amplifying and reproducing apparatus embodying electron discharge devices and means providing a source of alternating current for energizing certain of the operating circuits of said devices, of means for supplying alternating current from said source directly to the reproducing portion of said apparatus,

and means in circuit with the reproducing portion of said apparatus for varying the phase angle and magnitude of said alternating current.

11. In combination, an amplifier embodying electron discharge devices and having an output circuit, an alternating current supply circuit with which the filaments of said the amplifier output circuit whereby said out;

put circuit receives current directly from said supply circuit, said means being variable to vary the phase angle and magnitude of said current.

In witness whereof, I have hereunto set my hand this 9th day of May, 1928.

LOUIS THOMPSON. 

